When designing a visual slide projector show, which may or may not include audio effects as well, it is the practice to use multiple slide projectors, and it is the practice to fade one image on another to produce pleasing aesthetic visual effects. It is also the practice to provide for portions of one slide to be projected on the screen simultaneously with portions of another slide. Three, or up to thirty or more, image portions from different slides may be projected either simultaneously or sequentially. In these cases, it is also the practice to fade the portions of the different images in sequence so as again to provide a pleasing effect on the screen.
In order to avoid unsightly gaps or spaces, image portions from separate slides are projected simultaneously, with adjacent images overlapping. It is the practice to provide a "mask", masking out a portion of one of the images, so that a portion of the other image may be projected into the masked out portion. Similarly, the other slide will have image portions which are not masked out, so that only that portion is projected which will fit the masked out portion in the first image.
Essentially what is done is to create a "hole" in one image, and to create a portion of the other image which will fit the "hole", and overlap at the adjoining edges.
Where there are two portions of two different images overlapping on the same area of the screen at the same time the screen will be confused and may be excessively bright. This is undesirable and produces a sloppy result.
If it is attempted to reduce the marginal overlap to a minimum, it is difficult to align all of the projectors in the array of projectors required for a multiple image show so that the masks of different slides register precisely.
In order to overcome the problem, it has become common to use masks with blurred edges or what are known in the trade as "soft-edge masks". These masks have an image or shape in which the main portion of the mask area is either totally black, or totally transparent, and in which marginal areas represent a transition from black to transparent. The margins of the two masks overlap each other on the screen and reduce the illumination in the overlapped portion so as to equalize it with the illumination in the separate images. Such masks enable the producer of the slides to produce a multiple slide show in which the problem of registering the masked areas is less acute than when using hard edge masks.
However, such soft edge masks in the past have been manufactured by a somewhat rough and ready haphazard method, which resulted in an undesirable loss of sharpness or resolution of the shape of the mask. The process for such manufacture involved simply placing in front of a hard edge mask a panel of light diffuser material, such as translucent milky white acrylic plastic, or a panel of opalescent glass. The sharpness of the mask image is then blurred or diffused around the edges, and when photographed, produces a mask shape which is less well defined, and has a border area which gradually changes from black to transparent.
The use of a light diffuser panel results in a severe loss of resolution of the shape of the mask. When tested for resolution, it is found that these marginal portions have a resolution of less than 5 line pairs per millimeter. This is not a serious problem if the mask is a simple circle or oval. However, where the mask shape has angular corners, then the diffuser panel produces a severe distortion, such that the angular corners become rounded. This produces an unsightly result, when the two masks are projected on the screen in the manner described above. In addition, the use of a diffuser panel, in the manufacturing process caused some random scattering of light in the portion of the mask which was intended to be totally opaque. As a result soft edge masks produced by this method were lacking in contrast.
Other soft-edge masks have been made by simply photographing a "target" or object, with the camera lens out of focus. This however also produces loss of resolution and is unsatisfactory.
Another disadvantage of this type of soft edge mask is the fact that the transition from black to transparent, across the marginal area of such masks was not produced in a regular progressive fashion, since it was dependent simply on the scattering effect of the light diffuser panel. As a result, even if the two masks were registered more or less perfectly, the transitional area might exhibit some irregularities, with some patches being darker and others lighter.
In practice, however, perfect matching or registering of the two masks is virtually impossible, and some variation in the illumination of the overlapped area on the screen is noticeable, when using this type of mask.
Under ideal conditions, the transitional or marginal area of the mask, if examined from the totally black edge to the transparent edge, would exhibit a point more or less midway between the two edges which was capable of transmitting fifty percent of the light from that projector in which the mask was placed. In theory, this fifty percent point should be equidistant from the totally black edge, all the way around the shape of the mask. Assuming the two masks are perfectly registered, then this fifty percent point will coincide precisely on the screen, so that each of the two masks is transmitting fifty percent of the light from its projector at that point. The screen will thus receive fifty percent of the total projector illumination from the two projectors at that point. If this could be achieved, then the marginal area between the two images on the screen would be perfectly progressive from one image to the other and would be evenly illuminated. As noted above, however, since perfect registration is virtually impossible, there is almost always a certain degree of variation in the illumination of the overlapped area, which is difficult, if not impossible, to avoid, using current practice.
It is clearly desirable to provide a soft image mask which overcomes these various problems. In particular, such a soft image mask should avoid the loss of definition or resolution of the shape of the mask which is inherent in the use of a light diffuser panel.
Furthermore, would be desirable if the soft edge area of the mask could be produced in a precise and controlled manner, so that a controlled and predictable density would be produced from one edge to the other of the soft edge.
Furthermore, it is desirable that the soft edge be produced in such a way that the fifty percent point is essentially stretched or widened so that in spite of misregistration of the two masks, the appearance on the screen will nonetheless be virtually indistinguishable from a perfectly registered pair of masks.